test: add test cases for joins

Adds test cases for simple foreign keys, many to many tables, and joins
between tables. This tests the functionality of the commutator and
hash functions for the `ulid` type in postgres.

ref: [#13][issue]

[issue]: #13

src/lib.rs

@@ -194,6 +194,48 @@ mod tests {
         let result = Spi::get_one::<ulid>("SELECT gen_ulid();").unwrap();
         assert!(result.is_some());
     }
+
+    #[pg_test]
+    fn test_hash() {
+        Spi::run(
+            "CREATE TABLE foo (
+                id ulid,
+                data TEXT
+            );
+
+            CREATE TABLE bar (
+                id ulid,
+                foo_id ulid
+            );
+
+            INSERT INTO foo DEFAULT VALUES;
+            INSERT INTO bar DEFAULT VALUES;
+
+            SELECT *
+            FROM bar
+            JOIN foo ON bar.id = foo.id;",
+        )
+        .unwrap();
+    }
+
+    #[pg_test]
+    fn test_commutator() {
+        Spi::run(
+            "CREATE TABLE foo (
+                id ulid,
+                data TEXT
+            );
+
+            CREATE TABLE bar (
+                id ulid
+            );
+
+            SELECT *
+            FROM bar
+            JOIN foo ON bar.id = foo.id;",
+        )
+        .unwrap();
+    }
 }
 
 /// This module is required by `cargo pgrx test` invocations.